What is PSC Motor

A PSC motor (Permanent Split Capacitor Motor) is a single phase motor used in many HVAC (Heating, Ventilating and Air Conditioning) systems. A PSC motor uses a Run Capacitor in order to create the phase shift required to start the motor. However, unlike capacitor start motor – the start winding does not disconnect from the circuit after the motor has started. PSC motors are relatively easy to manufacture and highly reliable due to their simplistic construction. Apart from HVAC systems, PSC motors have proven to be very useful in other applications such as garage door openers, etc.

Apart from PSC motors, shaded pole motors are also popular in small HVAC applications. Click here if you are looking for C-frame shaded pole motors for the small ventilation and refrigeration fans

Construction:

The construction of a PSC motor is fairly simple. It comprises of two windings: a primary winding and an auxiliary winding. The auxiliary winding is connected through a capacitor which gives it a phase shift so that the magnetic fields from both windings do not cancel out each other. Since the capacitor is highly important to start a PSC motor, a faulty capacitor would mean the motor would completely stop. So in order to tackle regular wear and tear, the capacitor of the PSC motor exhibits self-healing characteristics due to inclusion of polypropylene, this reduces the failure rate of the motor preventing it from coming to halt. This happens because instead of discontinuing to work after getting damaged, the capacitor heals and continues to provide current to the auxiliary windings. However, it does deteriorates and lose its capacitance every time it heals. Inside this setup of windings, a rotor is placed. Its purpose is to start rotating as the motor is started. It is the only moving part in the entire motor along with the bearings. Apart from that there is circuitry to implement variable speeds by applying different resistances as you can see from the image below

The rotor of a PSC motor is enclosed using flanges. Depending upon the specification, some flanges have cooling holes to maintain the temperature of the motor and preventing a thermal problem.

Due to high speed rotation of the rotor some wear and tear is caused along with the heat generation due to friction. To minimize this problem, bearings are used to reduce the friction between flange and the rotor. Most PSC motors use Ball Bearings. As the name suggests, a ball bearing is a ring of steel balls which are placed around the rotor. These balls are free to roll and are lubricated using grease (As unlike oil, grease tend to stick to the surface and does not drift away at high speeds). These bearings are held in place around the rotor using a bowler ring attached to the flange. Since the bowler ring is attached to the flange, it does not rotate along with the bearings and remains stationary to keep the bearing balls held in the right place. In order to prevent air leakage from the motor, the bearing ring is kept at a minimum distance from the housing, causing an increased zero flow static pressure as compared to sleeve bearings used in other motors.

To prevent damage due to excessive heat generation, some manufacturers go even further and include thermal protectors in their motors. These thermal protectors are usually placed inside the circuitry to break the circuit in case they detect excessive heat inside the motor to prevent it from burning. After being triggered, many of the thermal protectors can only be reset manually. However, some of them resets automatically after the motor cools down. All this circuitry and components are then enclosed inside a housing to protect the parts from external environment and keep then intact.

Working:

When a PSC motor is switched on, current flows through both windings. Since the auxiliary winding is connected through a capacitor, it cause a phase shift in the current. This phase shift allows the magnetic field from both windings to add up instead of cancelling each other. The magnetic field causes flow of electrons in the rotor bars giving it a rotatory motion.

What is interesting is that if we remove the auxiliary winding from the circuit and try to start the motor, it won’t start. Instead the rotor would just vibrate and make a buzzing noise. This happens because – being equal and opposite in direction, the magnetic fields generated by the primary winding cancel out each other, giving zero rotatory force to the rotor. But if you go ahead and spin the rotor, the motor will start spinning because the force in one direction will become greater than the other. The auxiliary winding eliminates the need of mechanically starting the motor and that is why it is also called stator winding.

Advantages of a PSC Motor:

Because of their simplistic design and lesser number of breaking points, PSC motors are far more reliable than any of their alternatives.

They provide constant acceleration, so they are a very good choice for opening and closing garage doors.

Easily implementable mechanism for adding variable speeds.

PSC motors are highly efficient when they reach their top speed.

Since stator winding remains in the circuit even after the motor starts, no switch is required to take it out of the circuit (Unlike capacitor start motors).

Their rotatory motion can be easily reversed without any changes in their torque characteristics.

They are easier to manufacture (Again, thanks to their simple design) which leads to a very low failure rate. Add up a decent quality management and your chances of getting a lemon is extremely low.

PSC motors are very affordable when compared to their alternatives.

Due to all these factors mentioned above, they become the first choice of manufacturers to use in their products. So in case the motor of your HVAC system fails, there is a very high chance that you would need a PSC motor to replace that.

Due to being the first choice of manufacturers, they are the most common motors used in HVAC systems, making them readily available in the market. They come in so many sizes that you may find a PSC motor which fits in perfectly to replace any other motor of comparable specifications.